Fix E-commerce Site Quantity Selector Issues

am looking for a sharp front-end developer/JS expert to debug and fix a quantity selector on my e-commerce product pages. Everything worked flawlessly for months, but after a recent theme update, several frustrating bugs appeared... (Budget: $30 - $250 USD, Jobs: CSS, Debugging, Frontend Development, HTML, JavaScript, Mobile Development, PHP, Web Development, Website Design, Website Testing)